FAQ - How the Zero Cross function works on SSR's?

A: How the Zero Cross Function Works:

Omron SSRs with a zero cross function turns the relay ON when the AC load voltage is near the zero-voltage point (i.e., when the waveform crosses 0V). This reduces electrical noise and suppresses inrush current, leading to smoother, quieter, and more reliable operation, especially for resistive and capacitive loads. 

Ideally, the function turns ON near 0 V, but restrictions in the circuit configuration cause it to operate within the range of 0 V ±20 V. This voltage is called zero cross voltage.

Key Benefits:

Reduced Noise: This prevents the sudden rise in current that occurs when a load is switched ON at a high voltage point, which generates both conductive and radiated noise.

Controlled Switching: When the control signal is applied, the SSR delays the output to turn ON at the next available zero-voltage point, rather than immediately.

Lower Inrush Current: Prevents high initial currents, particularly important for heaters, lamps, and capacitive devices.

Longer Load Life: Reduces stress on the load by avoiding voltage peaks during switching.

Extended SSR Life: Protects internal switching elements, enhancing overall reliability.

Ideal for: Such as heaters, incandescent lamps, some types of motors, and when a quiet operation is needed for environments sensitive to electrical or audible noise (e.g., offices, homes, medical equipment).

Note:

For applications involving certain inductive loads (e.g., transformers or some motors), zero-cross SSRs may not be suitable. In these cases, a random turn-on SSR, which can switch at any point in the AC cycle, may provide better performance.

Date/Revision History

First review 9/22/2025 V1.0

Author: 

Aimee Rivera, Technical Support Engineer

References

Technical Explanation for Solid-state Relays